ci docs: clarify allow_failure default

doc/ci/yaml/README.md

@@ -492,6 +492,7 @@ osx job:
 
 `allow_failure` is used when you want to allow a job to fail without impacting
 the rest of the CI suite. Failed jobs don't contribute to the commit status.
+The default value is `false`.
 
 When enabled and the job fails, the pipeline will be successful/green for all
 intents and purposes, but a "CI build passed with warnings" message  will be